Resonator Electrode Shields

ABSTRACT

One embodiment of the present invention sets forth a MEMS resonator system that reduces interference signals arising from undesired capacitive coupling between different system elements. The system includes a MEMS resonator, two or more resonator electrodes, and at least one resonator electrode shield. The resonator electrode shield ensures that the resonator electrodes interact with either one or more shunting nodes or the active elements of the MEMS resonator by preventing or reducing, among other things, capacitive coupling between the resonator electrodes and the support and auxiliary elements of the MEMS resonator structure. By reducing the deleterious effects of interfering signals using one or more resonator electrode shields, a simpler, lower interference, and more efficient system relative to prior art approaches is presented.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 12/054,300,filed Mar. 24, 2008 (now U.S. Pat. No. 7,808,332), which is hereinincorporated by reference in its entirety. This application andapplication Ser. No. 12/054,300 claim benefit of U.S. Provisional PatentApplication Ser. No. 60/970,233, filed Sep. 5, 2007, which is alsoherein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate tomicroelectromechanical systems (MEMS) and, more specifically, toresonator electrode shields that are configured to reduce interferingsignals.

2. Description of the Related Art

Microelectromechanical system (MEMS) devices are currently beingdeveloped for a wide variety of applications. One such device is a MEMSresonator, which can be used in the timing circuitry of electronicdevices to generate a timing signal. MEMS resonator systems typicallyinclude one or more electrodes that drive the motion of a MEMS resonator(referred to herein as “drive electrodes”). As is well-known, when avoltage is applied to a drive electrode, a charge accumulates on theelectrode that applies an electrostatic force between the electrode andan opposite charge built up on the MEMS resonator structure. By applyinga time-varying voltage signals to the drive electrode, often incombination with a DC voltage, a time-varying electrostatic force can begenerated that capacitively couples mechanical energy to the activeelements of the MEMS resonator structure, causing the active elements toresonate (the term “active elements” is defined herein to be the one ormore elements of a the MEMS resonator structure whose motion causes theMEMS resonator to generate the desired timing signal). In addition oneor more sense electrodes generate or conduct a time-varying current as aresult of capacitive coupling between the moving active elements of theMEMS resonator structure and the sense electrodes. Ultimately, thetiming signal is extracted from the time-varying current conducted bythe sense electrodes. The sense and drive electrodes do not need to bepaired. For example, the number of sense and drive electrodes maydiffer, or the electrodes may be used for both sense and drive.Alternatively, the active elements can be used as the drive or senseelectrodes.

The desired capacitive coupling from the drive electrodes to the activeelements of the MEMS resonator structure to the sense electrodes may beaccompanied by additional coupling from drive to sense electrodesdirectly or through other elements of the MEMS resonator system. Thesesignal paths may create interfering signals that can compromise theoverall integrity of the generated timing signal.

One kind of an interfering signal, referred to herein as an “inducedcurrent,” can arise when time-varying voltage signals applied to thedrive electrodes causes time-varying currents to be induced within oneor more non-active elements of the MEMS resonator, support, or otherstructures (i.e., elements not responsible for generating the desiredtiming signal such as the elements that provide structural support forthe MEMS resonator system). These induced currents can alter the voltageon the MEMS resonator structure, which can capacitively couple to thesense electrodes. For example, when current is induced in supportstructures, these currents can cause voltages to be impressed on theresonator structures. The changing voltage on the resonator relative tothe sense electrodes can induce a sense current.

A second kind of an interfering signal, referred to herein as a“feed-through current,” can arise when the one or more drive electrodescouple capacitively directly to the one or more sense electrodes. Insuch a case a capacitively coupled signal can traverse past theresonator structure from drive to sense independently of the mechanicalmotion of the resonator.

Yet another kind of a interfering signal, referred to herein as a“spurious resonance current,” can arise due to the mechanical motion ofnon-active elements of the MEMS resonator structure. Such non-activeelements may include support or auxiliary elements within the overallMEMS resonator system. An electrostatic force from one or more driveelectrodes can cause one or more non-active elements, such as isolatingsprings, to move or resonate. The mechanical motion may be transduced bya nearby sense electrode, causing the sense electrode to conductadditional current, separate from the desired current produced inresponse to the desired motion of the active elements of the MEMSresonator structure.

There is additionally a form of spurious resonance current that can becaused from an undesired forcing of a resonant structure that excites anundesired mechanical mode. This differs from the previously describedspurious resonance in that it can be in the primary resonance structure,rather than for instance a support or an isolation spring. To avoidthis, the resonant structure in some cases must have a carefullycontrolled forcing and sensing geometry to assure that it only is exitedin a desired mode.

These spurious currents can decrease the signal quality of theoscillator that is built with the MEMS resonator. This can occur invarious ways that are particular to the type or cause of the currents.The descriptions below are examples and are not to be understood asexhaustive cases.

One consequence of the interfering currents can be reduced signal tonoise ratio (SNR) of an oscillator built with the MEMS resonator. Theinterfering currents can increase the electrical noise in the MEMSresonator system since they are not from the intended motion of theactive elements of the MEMS resonator structure.

Another consequence of interfering currents is decreased stability ofthe output frequency of an oscillator built with the MEMS resonator. Theinterfering currents can shift the phase of the MEMS resonator orsustaining circuit and shift the frequency of the oscillator. Forexample, feed-through current is normally 90° out of phase with the MEMSon-peak resonance current. When these currents are summed the aggregatecurrent is not in phase at the MEMS resonance peak. The oscillationcriteria can then only be satisfied if the circuitry's or theresonator's phase is shifted. This shift can change with temperature ortime and thereby affect the frequency stability.

Another consequence of interfering currents is that they may be largeenough to cause the overall oscillator to lock onto an undesirableresonant mode at an undesired frequency. For example, spurious currentscan excite an undesired resonance in a support or isolation structurethat can be mistaken for the desired resonance.

One way to minimize the interfering signals described herein is throughdifferential cancellation, where differential drive electrodes areconfigured to apply opposite charges on the MEMS resonator structure,and/or differential sense electrodes are configured to rejectcommon-mode coupled currents. However, not all electrodes and not allmechanical arrangements can be configured for differential signaling. Inaddition, there may be situations when one polarity of the differentialdrive and/or sense electrodes couple more strongly to the resonatorstructure than the other polarity. This can occur for instance when theelectrodes are not fully symmetrical. In such situation differentialcancellation may not be effective or may not be as effective as desired.

Another way to minimize interfering signals, particularly in the case ofinduced current is to decrease the induced voltage on the MEMS resonatorby decreasing the structural impedance between the MEMS resonator andeither the bias or ground. Such an approach may be implemented withcarefully designed electrical contacts, by increasing the doping withinthe MEMS resonator structure, and/or modifying the mechanical design ofthe MEMS resonator system. However, limitations on electrode design, themechanical and fabrication issues associated with achieving high dopinglevels within the MEMS resonator structure, and/or the mechanicalconstraints on the design place a limit on how much the overallimpedance may be decreased.

As the foregoing illustrates, what is needed in the art is a differentway to decrease the adverse effects of interfering signals in a MEMSresonator system.

SUMMARY OF THE INVENTION

One embodiment of the present invention sets forth a system forgenerating a timing signal. The system includes amicro-electromechanical system (MEMS) resonator having a firstnon-active element, a first active element, and a second active element,where, in operation, the first active element and the second activeelement are configured to resonate. The system also includes a firstdrive electrode configured to receive a first time-varying signal thatcauses the first active element to resonate, a first sense electrodeconfigured to conduct a second time-varying signal in response to themotion of the second active element, and a first resonator electrodeshield configured to reduce an interfering signal associated with thefirst non-active element.

One advantage of the disclosed system is that, among other things, theresonator electrode shield reduces undesired capacitive coupling betweenthe resonator electrodes and the non-active elements of the MEMSresonator. As a result, the deleterious effects of interfering signalsmay be mitigated, enabling the disclosed systems to produce higherquality timing signals in a manner that is simpler and more efficientrelative to prior art approaches. A second advantage is a reduction infeed-through current.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1A illustrates a MEMS resonator system without resonator electrodeshields, according to prior art;

FIG. 1B illustrates a MEMS resonator system with resonator electrodeshields, according to one embodiment of the present invention;

FIG. 2 illustrates resonator electrode shields implemented with a tuningfork-configured MEMS resonator system, according to another embodimentof the present invention;

FIGS. 3A-3E illustrate resonator electrode shields implemented with aquad-configured MEMS resonator system, according to differentembodiments of the present invention;

FIG. 4 illustrates a cross-sectional view of a portion of a MEMSresonator system, according to one embodiment of the present invention;

FIG. 5 illustrates differentially-coupled tuning fork-configured MEMSresonator system, according to yet another embodiment of the presentinvention;

FIG. 6A illustrates the effects of feedthrough interfering signals onthe frequency response of the signal generated by a MEMS resonator whenthe system does not include a resonator electrode shield;

FIG. 6B illustrates the effects of feedthrough interfering signals onthe frequency response of the signal generated by a MEMS resonator whenthe system does include a resonator electrode shield;

FIG. 7 is a conceptual diagram of a MEMS oscillator sustaining circuit,according to one embodiment of the present invention;

FIG. 8 is a conceptual diagram of an electronic device configured toimplement one or more aspects of the present invention; and

FIGS. 9A-9E illustrate various ways to position a MEMS resonator, adrive circuit, and application circuitry on one or more substrates,according to different embodiments of the present invention.

DETAILED DESCRIPTION

FIG. 1A illustrates a MEMS resonator system 100A without resonatorelectrode shields, according to prior art. As shown, the MEMS resonatorsystem 100A includes a MEMS resonator 110, a drive electrode 140A, and asense electrode 160A. The MEMS resonator further includes a MEMSresonator beam 111, a support element 120, and a flexure element 130.The MEMS resonator beam 111 is referred to herein as an “active element”of the MEMS resonator 110 because the motion of the MEMS resonator beam111 ultimately leads to the generation of the desired timing signal inthe MEMS resonator system 100A. The support element 120 and the flexureelement 130 are referred to herein as “non-active elements” of the MEMSresonator 110 because there elements are not supposed to be driven bythe drive electrode 140A and, therefore, should not contribute to thetiming signal produced by the MEMS resonator system 100A. The supportelement 120 is configured to provide an anchor 125 for the MEMSresonator 110 as well as mechanical support for the MEMS resonator beam111. The flexure element 130 is an auxiliary element configured to allowthe MEMS resonator beam 111 to resonate.

The drive electrode 140A may cause the MEMS resonator beam 111 toresonate in the direction indicated by arrow 151. In response, the senseelectrode 160A may generate or conduct a time-varying current as aresult of capacitive coupling between the MEMS resonator beam 111 andthe sense electrode 160A. Ultimately, the timing signal may be extractedfrom the time-varying current conducted by the sense electrode 160A.

The desired capacitive coupling from the drive electrode 140A to theMEMS resonator beam 111 may be accompanied by additional coupling fromthe drive electrode 140A to the sense electrode 160A either directly orthrough other elements of the MEMS resonator system 100A. As previouslydescried herein, these signal paths may create induced currents,feed-through currents, and spurious resonance currents that compromisethe overall integrity of the generated timing signal.

FIG. 1B illustrates a MEMS resonator system 100B with resonatorelectrode shields 171, 172, and 173, according to one embodiment of thepresent invention. Similarly to the MEMS resonator system 100A, the MEMSresonator system 100B includes a MEMS resonator 110, a drive electrode140B, and a sense electrode 160B. As shown, the resonator electrodeshield 171 is disposed between the non-active elements of the MEMSresonator 110 and the drive electrode 140B, the resonator electrodeshield 172 is disposed between the drive electrode 140B and the senseelectrode 160B, and the resonator electrode shield 173 is disposedbetween the non-active elements of the MEMS resonator 110 and the senseelectrode 160B. In one embodiment, the resonator electrode shields171-173 are fabricated from a conductive material, such as, for example,doped silicon. As a result, the electric field line between thenon-active elements of the MEMS resonator 110 and the resonatorelectrodes are terminated at the resonator electrode shields 171 and173, preventing or reducing capacitive coupling between the resonatorelectrodes on one side of the electrode shields and the non-activeelements of the MEMS resonator 110 on the other side. In addition, theresonator electrode shield 172 prevents or reduces capacitive couplingbetween the drive electrode 140B and the sense electrode 160B. Arrangingthe resonator electrode shields 171-173 in this fashion ensures that thedrive electrode 140B and the sense electrode 160B interact with only theactive element of the MEMS resonator 110 and are “shielded” fromcapacitively coupling to the non-active elements of the MEMS resonator110 and to each other.

Placing the resonator electrode shields 171-173 between the non-activeelements of the MEMS resonator 110, the drive electrode 140B, and thesense electrode 160B acts to decrease or prevent the interfering signalspreviously described herein. For example, without the resonatorelectrode shield 171, the time-varying voltage signal applied to thedrive electrode 140B could cause time-varying currents to be inducedwithin the support element 120 and/or the flexure element 130. Aspreviously described, these induced currents could alter the voltage onthe MEMS resonator 110. In turn, changing the voltage on the MEMSresonator 110 relative to the sense electrode 160B could induce acurrent in the sense electrode 160B, separate from the desired currentproduced in response to the desired motion of the active elements of theMEMS resonator 110. However, since the resonator electrode shield 171terminates the electric field lines between the drive electrode 140B andthe non-active elements of the MEMS resonator 110, the induced currentswithin the support element 120 and/or flexure element 130 are reduced orprevented, thereby preserving the constant voltage on the MEMS resonator110.

In addition, since the sense electrode 160B is located in closeproximity to the drive electrode 140B, without the resonator electrodeshield 172, the drive electrode 140B could capacitively couple directlyto the sense electrode 160B. Consequently, the sense electrode 160Bcould conduct not only the time-varying current based on the motion ofthe MEMS resonator beam 111, but could also conduct a feed-throughcurrent traversed past the MEMS resonator 110 to the sense electrode160B independently of the mechanical motion of the MEMS resonator beam111. However, the resonator electrode shield 172 terminates the electricfield lines between the drive electrode 140B and the sense electrode160B, thereby preventing or reducing the generation of feed-throughcurrent. In addition, placement of the resonator electrode shields171-173 may impact the tendency of the MEMS resonator 110 to resonate inunwanted modes (i.e., help to shape the force driving the modes). Forexample, in FIG. 1B, the portion of the resonator electrode shield 171that extends up to cover the bottom main portion of the MEMS resonatorbeam 111 will inhibit or partially inhibit the driving force on the pureside to side resonant mode while encouraging the preferred pendulum typeresonant mode.

Furthermore, during the operation of the MEMS resonator system 100B, themotion of the MEMS resonator beam 111 oftentimes cause both the supportelement 120 and the flexure element 130 to resonate. Without theresonator electrode shield 173, the motion of these non-active elementscould be detected by the nearby sense electrode 160B, causing the senseelectrode 160B to conduct additional spurious resonance current, aspreviously described herein. However, placing the resonator electrodeshield 173 between the sense electrode 160B and the non-active elementsof the MEMS resonator 110 prevents or reduces any capacitive couplingbetween the resonating non-active elements and the sense electrode 160Band, therefore, prevents or reduces the generation of the spuriousresonance current in the sense electrode 160B. In addition, reducing thelength of the drive electrode 140B so that the drive electrode 140B doesnot extend all the way to the non-active elements of the MEMS resonator110, and extending the resonator electrode shield 171, may reduce themotion of the support element 120 and the flexure element 130. As aresult, reduced motion may be transduced by the sense electrode 160B,diminishing or eliminating the spurious resonance currents.

FIG. 2 illustrates resonator electrode shields 271 and 272 implementedwith a tuning fork-configured MEMS resonator system 200, according toanother embodiment of the present invention. As shown, the MEMSresonator system 200 includes, without limitation, a MEMS resonator 210,drive electrodes 241 and 242, and a sense electrode 261. The MEMSresonator 210 further includes MEMS resonator beams 211 and 212, a leftelement 221, and a right element 222. In this configuration, the MEMSresonator beams 211 and 212 are referred the “active elements” of theMEMS resonator 210 because their motion ultimately leads to thegeneration of the desired timing signal in the MEMS resonator system200. The left element 221 and the right element 222 are the “non-activeelements” of the MEMS resonator 210 because these elements arestructural elements that are not supposed to contribute to the timingsignal produced by the MEMS resonator system 200.

In operation, the drive electrodes 241 and 242 cause the MEMS resonatorbeams 211 and 212 to resonate in the directions indicated by arrows 251and 252, respectively. The sense electrode 261 is paired to both of thedrive electrodes 241 and 242 and is located such that the MEMS resonatorbeam 211 is disposed between the sense electrode 261 and drive electrode241, and the MEMS resonator beam 212 is disposed between the senseelectrode 261 and drive electrode 242. The sense electrode 261 isconfigured to conduct a time-varying current resulting from thecapacitive coupling that occurs between the MEMS resonator beams 211 and212 as the MEMS resonator beams resonate.

As also shown, the MEMS resonator system 200 includes the resonatorelectrode shields 271 and 272 disposed between the non-active elementsof the MEMS resonator 210 and the sense electrode 261. Similar to theresonator electrode shields 171-173 of FIG. 1B, the resonator electrodes271 and 272 are fabricated from a conductive material (e.g., dopedsilicon). Arranging the resonator electrode shields 271 and 272 in thisfashion ensures that the sense electrode 261 interacts with only theMEMS resonator beams 211 and 212 and is “shielded” from capacitivelycoupling to the left element 221 and the right element 222.

Again, placing the resonator electrode shields 271 and 272 between thenon-active elements of the MEMS resonator 210 and the sense electrode261 acts to prevent or reduce the interfering signals previouslydescribed herein. For example, the time-varying voltage signal appliedto the drive electrodes 241 and 242 could cause time-varying currents tobe induced within the left element 221 and the right element 222 of theMEMS resonator 210. Since the sense electrode 261 is located in closeproximity to the left element 221 and the right element 222, without theresonator electrode shields 271 and 272, the time-varying inducedcurrents within the left element 221 and the right element 222 could be“sensed” by the sense electrode 261. In such a case, the sense electrode261 could conduct an additional current based on the induced currents inthe non-active elements of the MEMS resonator 210, separate from thedesired current produced in response to the desired motion of the MEMSresonator beams 211 and 212. However, the resonator electrode shields271 and 272 prevent or reduce the sense electrode 261 from “sensing” thecurrents induced within the non-active elements.

Furthermore, during the operation of the MEMS resonator system 200, themotion of the MEMS resonator beams 211 and 212 could cause the leftelement 221 and the right element 222 to resonate. Without the resonatorelectrode shields 271 and 272, the motion of these non-active elementscould be detected by the sense electrode 261, causing the senseelectrode 261 to conduct spurious resonance currents, as previouslydescribed herein. However, placing the resonator electrode shields 271and 272 between the sense electrode 261 and left element 221 and theright element 222 prevents or reduces any capacitive coupling betweenthe resonating non-active elements and the sense electrode 261 and,therefore, prevents or reduces the generation of the spurious resonancecurrents in the sense electrode 261.

FIG. 3A illustrates resonator electrode shields 371-374 implemented witha quad-configured MEMS resonator system 300A, according to oneembodiment of the present invention. As shown, the MEMS resonator system300A includes, without limitation, a MEMS resonator 310 and a number ofdrive electrodes 341A-344A and sense electrodes 361A-364A, disposed indifferent locations relative to the MEMS resonator 310. The senseelectrodes 361A and 363A and the drive electrodes 342A and 344A arelocated inside the MEMS resonator 310. The sense electrodes 362A and364A and the drive electrodes 341A and 343A are located outside the MEMSresonator 310.

The MEMS resonator 310 further includes MEMS resonator beams 311-314, asupport element 320 that includes support arms 321-324, and isolatingspring elements 331-334. The MEMS resonator beams 311-314 are referredto herein as “active elements” of the MEMS resonator 310 because theirmotion ultimately leads to the generation of the desired timing signalin the MEMS resonator system 300A. The support element 320 and theisolating spring elements 331-334 are referred to herein as “non-activeelements” of the MEMS resonator 310 because these elements are notsupposed to be driven by the drive electrodes 341A-344A and, therefore,should not contribute to the timing signal produced by the MEMSresonator system 300A. The support element 320 is configured to providean anchor 325 for the MEMS resonator 310 as well as mechanical supportfor the MEMS resonator beams 311-314. The isolating spring elements331-334 are auxiliary elements that are configured to minimize thestress on the support arms 321-324 of the support element 320 caused bythe motion of the MEMS resonator beams 311-314.

The drive electrode 341A causes the MEMS resonator beam 311 to oscillatein the direction indicated by arrow 351. Similarly, the drive electrode342A causes the MEMS resonator beam 312 to oscillate in the directionindicated by arrow 352, the drive electrode 343A causes the MEMSresonator beam 313 to oscillate in the direction indicated by arrow 353,and the drive electrode 344A causes the MEMS resonator beam 314 tooscillate in the direction indicated by arrow 354. In thisconfiguration, for each of the drive electrodes 341A-344A, there is apaired sense electrode 361A-364A, respectively, arranged such that oneof the MEMS resonator beams 311-314 is disposed between the driveelectrode and sense electrode of each electrode pair. In operation, eachsense electrode is configured to conduct a time-varying currentresulting from the capacitive coupling that occurs between the MEMSresonator beam, disposed between each electrode pair, and the senseelectrode of that pair as the MEMS resonator beam oscillates. Personsskilled in the art will recognize that, in various embodiments, thedrive and sense electrodes do not necessarily need to be paired. Ingeneral, each of the drive electrode 341A-344A may comprise M driveelectrodes, and each of the sense electrodes 361A-364A may comprise Nsense electrodes. In such a configuration, each of the MEMS resonatorbeams 311-314 may be disposed between M drive electrodes and N senseelectrodes, where the M drive electrodes cause the MEMS resonator beamto oscillate, while the N sense electrodes conduct a time-varyingcurrent resulting from the capacitive coupling between the MEMSresonator beam and the sense electrodes.

As also shown, the MEMS resonator system 300A includes the resonatorelectrode shields 371-374 disposed between the non-active elements ofthe MEMS resonator 310 and the resonator electrodes located within theMEMS resonator 310. As shown, the resonator electrode shield 371 isdisposed between the sense electrode 361A on one side and the supportelement 320 and the isolating spring elements 331 and 332 on the otherside. Similarly, the resonator electrode shield 372 is disposed betweenthe drive electrode 342A on one side and the support element 320 and theisolating spring elements 332 and 333 on the other side. The resonatorelectrode shield 373 is disposed between the sense electrode 363A on oneside and the support element 320 and the isolating spring elements 333and 334 on the other side. Finally, the resonator electrode shield 374is disposed between the drive electrode 344A on one side and the supportelement 320 and the isolating spring elements 334 and 331 on the otherside. Similar to FIGS. 1B and 2, in one embodiment, the resonatorelectrode shields 371-374 are fabricated from a conductive material(e.g., doped silicon). As a result, the electric field lines between thenon-active elements of the MEMS resonator 310 and the resonatorelectrodes located inside the MEMS resonator 310 are terminated at theresonator electrode shields 371-374, preventing capacitive couplingbetween the resonator electrodes on one side of the electrode shieldsand the non-active elements of the MEMS resonator 310 on the other side.Arranging the resonator electrode shields 371-374 in this fashionensures that the drive electrodes 342A and 344A and the sense electrodes361A and 363A interact with only the active elements of the MEMSresonator 310 corresponding to these electrodes and are “shielded” fromcapacitively coupling to the non-active elements of the MEMS resonator310.

Again, placing the resonator electrode shields 371-374 between thenon-active elements of the MEMS resonator 310 and the resonatorelectrodes located within the MEMS resonator 310 acts to prevent theinterfering signals previously described herein. For example, withoutthe resonator electrode shields 372 and 374, the time-varying voltagesignal applied to the drive electrodes 342A and 344A could causetime-varying currents to be induced within the support arms 321-324 ofthe MEMS resonator 310. As previously described, these induced currentscould alter the voltage on the MEMS resonator 310, which may change themagnitude of the electrostatic forces between the MEMS resonator beams312 and 314 and the drive electrodes 342A and 344A, thereby negativelyimpacting the generated timing signal. However, since the resonatorelectrode shields 372 and 374 terminate the electric field lines fromthe drive electrodes 342A and 344A, the induced currents within thesupport arms 321-324 are prevented, thereby preserving the constantvoltage on the MEMS resonator 310.

In addition, since the sense electrodes 361A and 363A are located inclose proximity to the support arms 321-324, without the resonatorelectrode shields 371-374, the time-varying induced currents within thesupport arms 321-324 could be “sensed” by the sense electrodes 361A and363A. Consequently, the sense electrodes 361A and 363A could conduct notonly the time-varying current based on the motion of the MEMS resonatorbeams 311 and 313, respectively, but could also conduct an additionalcurrent based on the induced current within the support arms 321-324caused by the unpaired drive electrodes 342A and 344A. However, sincethe resonator electrode shields 372 and 374 prevent the induced currentswithin the support arms 321-324, the shields also prevent the generationof the additional currents based on the induced currents in thenon-active elements.

Furthermore, during the operation of the MEMS resonator system 300A, theoscillations of the MEMS resonator beams 311-314 oftentimes cause boththe support arms 321-324 and the isolating spring elements 331-334 tovibrate. Without the resonator electrode shields 371 and 373, themechanical vibrations of these non-active elements could be transducedby the nearby sense electrodes 361A and 363A, causing the senseelectrodes 361A and 363A to conduct spurious resonance currents, aspreviously described herein. However, placing the resonator electrodeshields 371 and 373 between the sense electrodes 361A and 363A and thenon-active elements of the MEMS resonator 310 prevents any capacitivecoupling between the vibrating non-active elements and the senseelectrodes and, therefore, prevents the generation of spurious resonancecurrents in the sense electrodes 361A and 363A.

FIG. 3B illustrates resonator electrode shields 371-374 implemented witha quad-configured MEMS resonator system 300B, according to anotherembodiment of the present invention. The MEMS resonator system 300Bdiffers from the MEMS resonator system 300A by the locations of driveelectrodes 341B-344B and sense electrodes 361B-364B relative to the MEMSresonator 310. Similar to the explanation associated with FIG. 3A, invarious embodiments, the drive and sense electrodes do not necessarilyneed to be paired. In general, each of the drive electrode 341B-344B maycomprise M drive electrodes, and each of the sense electrodes 361B-364Bmay comprise N sense electrodes. Again, in such a configuration, each ofthe MEMS resonator beams 311-314 may be disposed between M driveelectrodes and N sense electrodes, where the M drive electrodes causethe MEMS resonator beam to oscillate, while the N sense electrodesconduct a time-varying current resulting from the capacitive couplingbetween the MEMS resonator beam and the sense electrodes.

As shown, in the embodiment of FIG. 3B, all of the sense electrodes361B-364B are disposed outside the MEMS resonator 310 and all of thedrive electrodes 341B-344B are disposed inside the MEMS resonator 310.Persons skilled in the art will recognize that the location of theresonator electrodes relative to the MEMS resonator 310 affects whichtypes of interfering signals are generated within the system. Here,since the sense electrodes 361B-364B are located outside of the MEMSresonator 310, the mechanical vibrations of non-active elements withinthe MEMS resonator 310 generally do not cause the sense electrodes361B-364B to conduct spurious resonance currents.

However, since the time-varying voltage signal applied to all of thedrive electrodes 341B-344B could cause electrical currents to be inducedwithin the support arms 321-324 of the MEMS resonator 310, these inducedcurrents could alter the voltage on the MEMS resonator 310. Alteredvoltages on the active elements of the MEMS resonator 310 could, inturn, capacitively induce additional currents in the sense electrodes361B-364B. As a result, the sense electrodes 361B-364B not only couldconduct the time-varying current based on the desired motion of the MEMSresonator beams 311-314, but also could conduct an additional currentbased on the induced current within the support arms 321-324 caused bythe drive electrodes 341B-344B. Similar to FIG. 3A, the resonatorelectrode shields 371-374 terminate the electric field lines from thedrive electrodes 341B-344B, thereby preventing the induced currentswithin the support arms 321-324. Consequently, the desired constantvoltage on the MEMS resonator 310 may be preserved, preventing thegeneration of additional currents in the sense electrodes 361B-364Bbased on the induced currents in the non-active elements.

FIG. 3C illustrates resonator electrode shields 371-374 implemented witha quad-configured MEMS resonator system 300C, according to anotherembodiment of the present invention. Again, the MEMS resonator system300C differs from the MEMS resonator systems 300A and 300B by thelocations of drive electrodes 341C-344C and sense electrodes 361C-364Crelative to the MEMS resonator 310. Similar to the explanationassociated with FIG. 3A, in various embodiments, the drive and senseelectrodes do not necessarily need to be paired. In general, each of thedrive electrode 341C-344C may comprise M drive electrodes, and each ofthe sense electrodes 361C-364C may comprise N sense electrodes. Again,in such a configuration, each of the MEMS resonator beams 311-314 may bedisposed between M drive electrodes and N sense electrodes, where the Mdrive electrodes cause the MEMS resonator beam to oscillate, while the Nsense electrodes conduct a time-varying current resulting from thecapacitive coupling between the MEMS resonator beam and the senseelectrodes.

As shown, in this embodiment, all of the sense electrodes 361C-364C aredisposed inside the MEMS resonator 310, and all of the drive electrodes341C-344C are disposed outside the MEMS resonator 310. Again, thelocation of the resonator electrodes relative to the MEMS resonator 310affects which of the interfering signals are generated within thesystem. Here, even though the drive electrodes 341C-344C are locatedoutside of the MEMS resonator 310, the drive electrodes may still causetime-varying currents to be induced within the support arms 321-324 bycapacitively coupling to the outer edges of the support arms 321-324.Further, since the sense electrodes 361C-364C are located in closeproximity to the support arms 321-324, without the resonator electrodeshields 371-374, the time-varying induced currents within the supportarms 321-324 could be “sensed” by the sense electrodes 361C-364C.Consequently, the sense electrodes 361C-364C not only could conduct thetime-varying current based on the motion of the MEMS resonator beams311-314, but also could conduct an additional current based on theinduced current within the support arms 321-324 caused by the driveelectrodes 341C-344C. The resonator electrode shields 371-374 terminatethe electric field lines from the support arms 321-324, therebypreventing the generation of the additional currents based on theinduced currents in the non-active elements.

In addition, similar to FIG. 3A, during the operation of the MEMSresonator system 300C, the oscillations of the MEMS resonator beams311-314 oftentimes cause both the support arms 321-324 and the isolatingspring elements 331-334 to vibrate. The mechanical vibrations of thesenon-active elements of the MEMS resonator could be detected by thenearby sense electrodes 361C-364C, causing the sense electrodes361C-364C to conduct spurious resonance currents, as previouslydescribed herein. Placing the resonator electrode shields 371-374between the sense electrodes 361C-364C and the non-active elements ofthe MEMS resonator 310 prevents any capacitive coupling between thevibrating non-active elements and the sense electrodes and, therefore,prevents the generation of spurious resonance currents in the senseelectrodes 361C-364C.

FIG. 3D illustrates resonator electrode shields 371-374 implemented witha quad-configured MEMS resonator system 300D, according to anotherembodiment of the present invention. Again, the MEMS resonator system300D differs from the MEMS resonator systems 300A-300C by the locationsof drive electrodes 341D-344D and sense electrodes 361D-364D relative tothe MEMS resonator 310. In this embodiment, the drive electrodes341D-344D and the sense electrodes 361D-364D are differentially coupled.Thus, the sense electrodes 361D and 362D are disposed on opposite sidesof the MEMS resonator beam 311, with one of the sense electrodes 361Dand 362D being the positive electrode, and the other electrode being thenegative electrode. The sense electrodes 363D and 364D are disposed onthe opposite sides of the MEMS resonator beam 312, with one of the senseelectrodes 363D and 364D being the positive electrode, and the otherelectrode being the negative electrode. The drive electrodes 341D and342D are disposed on the opposite sides of the MEMS resonator beam 313,with one of the drive electrodes 341D and 342D being the positiveelectrode, and the other electrode being the negative electrode.Finally, the drive electrodes 343D and 344D are disposed on the oppositesides of the MEMS resonator beam 314, with one of the drive electrodes343D and 344D being the positive electrode, and the other electrodebeing the negative electrode.

In various embodiments, each of the drive electrode 341D-344D maycomprise any technically feasible number of drive electrodes, and eachof the sense electrodes 361D-364D may comprise any technically feasiblenumber of sense electrodes. In such configurations, each of the MEMSresonator beams 311-314 may be disposed between M electrodes on one sideand N electrodes on the other side.

Again, placing the resonator electrode shields 371-374 between thenon-active elements of the MEMS resonator 310 and the resonatorelectrodes located within the MEMS resonator 310 acts to prevent theinterfering signals previously described herein. For example, withoutthe resonator electrode shields 373 and 374, the time-varying voltagesignal applied to the drive electrodes 341D-344D could causetime-varying currents to be induced within the support arms 321-324 ofthe MEMS resonator 310. As previously described, these induced currentscould alter the voltage on the MEMS resonator 310, which may change themagnitude of the electrostatic forces between the MEMS resonator beams313 and 314 and the drive electrodes 341D-344D, thereby negativelyimpacting the generated timing signal. However, since the resonatorelectrode shields 373 and 374 terminate the electric field lines fromthe drive electrodes 341D-344D, the induced currents within the supportarms 321-324 are prevented, thereby preserving the constant voltage onthe MEMS resonator 310.

In addition, since the sense electrodes 361D-364D are located in closeproximity to the support arms 321-324, without the resonator electrodeshields 371-374, the time-varying induced currents within the supportarms 321-324 could be “sensed” by the sense electrodes 361D-364D.Consequently, the sense electrodes 361D-364D could conduct not only thetime-varying current based on the motion of the MEMS resonator beams 311and 312, but could also conduct an additional current based on theinduced current within the support arms 321-324 caused by the driveelectrodes 341D-344D. Again, since the resonator electrode shields 373and 374 prevent the induced currents within the support arms 321-324,and the resonator electrode shields 371 and 372 further prevent theinduced currents being sensed by the sense electrodes 361D-364D, theshields also prevent the generation of the additional currents based onthe induced currents in the non-active elements.

Furthermore, during the operation of the MEMS resonator system 300D, theoscillations of the MEMS resonator beams 311-314 may cause both thesupport arms 321-324 and the isolating spring elements 331-334 tovibrate. Without the resonator electrode shields 371 and 372, themechanical vibrations of these non-active elements could be transducedby the nearby sense electrodes 362D and 364D, causing the senseelectrodes 362D and 364D to conduct spurious resonance currents, aspreviously described herein. However, placing the resonator electrodeshields 371 and 372 between the sense electrodes 362D and 364D and thenon-active elements of the MEMS resonator 310 prevents any capacitivecoupling between the vibrating non-active elements and the senseelectrodes and, therefore, prevents the generation of spurious resonancecurrents in the sense electrodes 362D and 364D.

The resonator electrode shield 371 also facilitates balancing theparasitic capacitances sensed by the sense electrodes 361D and 362D.Similarly, the resonator electrode shield 372 further facilitatesbalancing the parasitic capacitances sensed by the sense electrodes 363Dand 364D. Without the resonator electrode shields 371 and 372, parasiticcapacitances sensed by the sense electrodes disposed inside the MEMSresonator 310 (i.e., the sense electrodes 362D and 364D) oftentimes isdifferent than the parasitic capacitances sensed by the sense electrodesdisposed outside of the MEMS resonator 310 (i.e., the sense electrodes361D and 363D).

The resonator electrode shields 373 and 374 also prevent drive-to-drivecoupling between the drive electrode 342D and the drive electrode 344D.Without the resonator electrode shields 373 and 374, the driveelectrodes 342D and 344D may capacitively couple to each other via thenon-active elements of the MEMS resonator 310. Placing the resonatorelectrode shields 373 and 374 between the drive electrodes 342D and 344Dand the non-active elements of the MEMS resonator 310 eliminatescapacitive coupling between the drive electrodes and the non-activeelements and, therefore, prevents the drive-to-drive coupling.

FIG. 3E illustrates resonator electrode shields 371-374 implemented witha quad-configured MEMS resonator system 300E, according to yet anotherembodiment of the present invention. Again, the MEMS resonator system300E differs from the MEMS resonator systems 300A-300D by the locationsof drive electrodes 341E-344E and sense electrodes 361E-364E relative tothe MEMS resonator 310. Similar to FIG. 3D, in this embodiment, thedrive electrodes 341E-344E and the sense electrodes 361E-364E are alsodifferentially coupled. Thus, the sense electrodes 361E and 362E aredisposed on opposite sides of the MEMS resonator beam 311, with one ofthe sense electrodes 361E and 362E being the positive electrode, and theother electrode being the negative electrode. Similarly, the senseelectrodes 363E and 364E are disposed on the opposite sides of the MEMSresonator beam 313, with one of the sense electrodes 363E and 364E beingthe positive electrode, and the other electrode being the negativeelectrode. The drive electrodes 341E and 342E are disposed on theopposite sides of the MEMS resonator beam 312, with one of the driveelectrodes 341E and 342E being the positive electrode, and the otherelectrode being the negative electrode. Finally, the drive electrodes343E and 344E are disposed on the opposite sides of the MEMS resonatorbeam 314, with one of the drive electrodes 343E and 344E being thepositive electrode, and the other electrode being the negativeelectrode.

In various embodiments, each of the drive electrode 341E-344E maycomprise any technically feasible number of drive electrodes, and eachof the sense electrodes 361E-364E may comprise any technically feasiblenumber of sense electrodes. In such configurations, each of the MEMSresonator beams 311-314 may be disposed between M electrodes on one sideand N electrodes on the other side.

Again, placing the resonator electrode shields 371-374 between thenon-active elements of the MEMS resonator 310 and the resonatorelectrodes located within the MEMS resonator 310 acts to prevent theinterfering signals previously described herein. For example, withoutthe resonator electrode shields 372 and 374, the time-varying voltagesignal applied to the drive electrodes 341E-344E could causetime-varying currents to be induced within the support arms 321-324 ofthe MEMS resonator 310. As previously described, these induced currentscould alter the voltage on the MEMS resonator 310, which may change themagnitude of the electrostatic forces between the MEMS resonator beams312 and 314 and the drive electrodes 341E-344E, thereby negativelyimpacting the generated timing signal. However, since the resonatorelectrode shields 372 and 374 terminate the electric field lines fromthe drive electrodes 341E-344E, the induced currents within the supportarms 321-324 are prevented, thereby preserving the constant voltage onthe MEMS resonator 310.

In addition, since the sense electrodes 361E-364E are located in closeproximity to the support arms 321-324, without the resonator electrodeshields 371-374, the time-varying induced currents within the supportarms 321-324 could be “sensed” by the sense electrodes 361E-364E.Consequently, the sense electrodes 361E-364E could conduct not only thetime-varying current based on the motion of the MEMS resonator beams 311and 313, but could also conduct an additional current based on theinduced current within the support arms 321-324 caused by the driveelectrodes 341E-344E. Again, since the resonator electrode shields 372and 374 prevent the induced currents within the support arms 321-324,and the resonator electrode shields 371 and 373 further prevent theinduced currents being sensed by the sense electrodes 361E-364E, theshields also prevent the generation of the additional currents based onthe induced currents in the non-active elements.

Furthermore, during the operation of the MEMS resonator system 300E, theoscillations of the MEMS resonator beams 311-314 may cause both thesupport arms 321-324 and the isolating spring elements 331-334 tovibrate. Without the resonator electrode shields 371 and 373, themechanical vibrations of these non-active elements could be transducedby the nearby sense electrodes 362E and 364E, causing the senseelectrodes 362E and 364E to conduct spurious resonance currents, aspreviously described herein. However, placing the resonator electrodeshields 371 and 373 between the sense electrodes 362E and 364E and thenon-active elements of the MEMS resonator 310 prevents any capacitivecoupling between the vibrating non-active elements and the senseelectrodes and, therefore, prevents the generation of spurious resonancecurrents in the sense electrodes 362E and 364E.

The resonator electrode shield 371 also facilitates balancing theparasitic capacitances sensed by the sense electrodes 361E and 362E.Similarly, the resonator electrode shield 373 further facilitatesbalancing the parasitic capacitances sensed by the sense electrodes 363Eand 364E. Without the resonator electrode shields 371 and 373, parasiticcapacitances sensed by the sense electrodes disposed inside the MEMSresonator 310 (i.e., the sense electrodes 362E and 364E) oftentimes isdifferent than the parasitic capacitances sensed by the sense electrodesdisposed outside of the MEMS resonator 310 (i.e., the sense electrodes361E and 363E).

The resonator electrode shields 372 and 374 also prevent drive-to-drivecoupling between the drive electrode 342E and the drive electrode 344E.Without the resonator electrode shields 372 and 374, the driveelectrodes 342E and 344E may capacitively couple to each other via thenon-active elements of the MEMS resonator 310. Placing the resonatorelectrode shields 372 and 374 between the drive electrodes 342E and 344Eand the non-active elements of the MEMS resonator 310 eliminatescapacitive coupling between the drive electrodes and the non-activeelements and, therefore, prevents the drive-to-drive coupling.

FIG. 4 illustrates a cross-sectional view of a portion of a MEMSresonator system 400, according to one embodiment of the presentinvention. As shown, the MEMS resonator system 400 includes, withoutlimitation, a support element 420, a MEMS resonator beam 480, a firstresonator electrode 481, a second resonator electrode 482, oxide 487,metal 488, and encapsulation 489. Similar to the systems of FIGS. 3A-3E,the support element 420 is the non-active element of the MEMS resonatorsystem 400. In various embodiments, the first resonator electrode 481could be either a drive or a sense electrode. The second resonatorelectrode 482 is then a corresponding sense or drive electrode, as thecase may be. The MEMS resonator beam 480 is disposed between the firstresonator electrode 481 and the second resonator electrode 482. The MEMSresonator beam 480 is one of the active elements of the MEMS resonatorsystem 400 because, as previously described, the motion of the MEMSresonator beam 480 ultimately leads to the generation of the desiredtiming signal in the MEMS resonator system 400.

As also shown, the MEMS resonator system 400 includes a resonatorelectrode shield 485 disposed between the support element 420 and thefirst resonator electrode 481. If the resonator electrode shield 485were removed from the disclosed structure, then one or more of theinterfering signals described in FIGS. 3A-3E may result. For example, anunpaired electrode (not shown) could capacitively couple to theresonator electrode 481 through the support element 420, and/or thefirst resonator electrode 481 could induce a time-varying current in thesupport element 420, as indicated by dashed line 491. However, since theresonator electrode shield 485 terminates the electric field lines toand from the first resonator electrode 481, capacitive coupling may onlyoccur between the support element 420 and the resonator electrode shield485, as indicated by dashed line 492, and between the resonatorelectrode shield 485 and the first resonator electrode 481, as indicatedby dashed line 493. Moreover, the resonator electrode shield 485 isconnected to the ground source, as indicated by arrow 495, thus safelycarrying away accumulated electrical charge, as described in more detailbelow, which prevents or reduces the induced currents within the supportelement 420.

FIG. 5 illustrates a differentially-coupled tuning fork-configured MEMSresonator system 500, according to yet another embodiment of the presentinvention. The MEMS resonator system 500 includes a MEMS resonator 510,differential drive electrodes 541 and 542, differential sense electrodes561 and 562, and a resonator electrode shield 571 disposed between thedrive electrode 542 and the sense electrode 561. The MEMS resonator 510further includes MEMS resonator beams 511 and 512, a support element520, including an anchor 525, and a flexure element 530. With such aconfiguration, the charges do not have to necessarily come through thesupport element 520 or out of the anchor 525, some can travel just theshort distance between the MEMS resonator beams 511 and 512. Since thedrive electrode 542 is located in close proximity to the sense electrode561, the drive electrode 542 could capacitively couple directly to thesense electrode 561. Consequently, the sense electrode 561 could conductnot only the time-varying current based on the motion of the MEMSresonator beams 511 and 512, but could also conduct a feed-throughcurrent traversed past the MEMS resonator 510 to the sense electrode 561independently of the mechanical motion of the MEMS resonator beams 511and 512. However, the resonator electrode shield 571 terminates theelectric field lines between the drive electrode 542 and the senseelectrode 561, thereby preventing or reducing the generation offeed-through current.

As previously described herein, the existence of interfering signalswithin a MEMS resonator system impairs the integrity of the timingsignal being generated by that system. FIG. 6A illustrates the effectsof interfering signals on the frequency response of the signal generatedby a MEMS resonator when the system does not include resonator electrodeshields. As shown, a solid line 600A represents the desired frequencyresponse of the signal generated by a MEMS resonator, such as the MEMSresonator 110, 210, 310, or 510, and frequency f_(o) represents theresonant frequency of the MEMS resonator. Point 610A illustrates thesignal strength of the frequency response at the resonant frequency.When the system does not include the resonator electrode shields,obtaining the frequency response depicted by line 600 becomes moredifficult because interfering signals are increased relative to thedesired signal due to the induced, feed-through, and spurious resonancecurrents generated in the system. As shown by lines 602, 604, 606 and608, as the interfering signals increase, the desired signal response atthe resonant frequency f_(o) becomes more heavily masked.

FIG. 6B illustrates the effects of interfering signals on the frequencyresponse of the signal generated by a MEMS resonator when the systemincludes resonator electrode shields. As shown, a solid line 600Arepresents the frequency response of the signal generated by a MEMSresonator, such as the MEMS resonator 110, 210, 310, or 510 andfrequency f₀ represents the resonant frequency of the MEMS resonator.Point 610B illustrates the signal strength of the frequency response atthe resonant frequency. The resonator electrode shields reduce theinterfering signals generated by the induced, feed-through, and spuriousresonance currents. Consequently, the signal strength at the resonantfrequency f₀ relative to the interfering signal becomes greater.

There are several design guidelines applicable to the resonatorelectrode shields illustrated in FIGS. 1B, 2, 3A-3E, and FIG. 5. A firstguideline is that none of the resonant frequencies of the resonatorelectrode shield should match any of the resonant frequencies of theactive elements of the MEMS resonator. Moreover, harmonics of both theMEMS resonator and the resonator electrode shield should also not match.Persons skilled in the art will recognize that a match in resonantfrequencies would result in the excitation of the resonator electrodeshield. In order to avoid such a situation, the material properties andgeometry of the resonator electrode shields should be chosenaccordingly.

A second guideline is that the resonator electrode shields should beelectrically coupled to an electrical sink. This guideline arises fromthe fact that, during the operation, resonator electrode shieldsaccumulate charge, and this charge could be injected into the MEMSresonator, compromising the overall performance of the MEMS resonatorsystem. By electrically coupling the resonator electrode shields to anelectrical sink, the accumulated charge may be safely carried away fromthe MEMS resonator. The resonator electrode shields may be coupled tothe same potential as the field area, for example, to the bias or groundsources.

Another guideline is that, during the operation, the resonator electrodeshields should be non-depleted or only be partially depleted of mobilecarriers. If a resonator electrode shield is completely depleted ofmobile carriers, then the shield could cease to be conductive, whichwould impede the shield's ability to terminate electric field lines.Therefore, the resonator electrode shields should be made thick enoughso that the complete depletion of mobile carriers may be avoided.

Yet another guideline is that the geometry of the resonator electrodeshields should render the shields mechanically stable, regardless of howthe shields are affixed within the MEMS resonator system.

FIG. 7 is a conceptual diagram of a MEMS oscillator sustaining circuit700, according to one embodiment of the present invention. As shown, theMEMS oscillator sustaining circuit 700 includes, without limitation, adrive circuit 720 used to drive the MEMS resonator system 100B andproduce an output signal 790. Alternatively, in place of the MEMSresonator system 100B, MEMS oscillator sustaining circuit 700 mayinclude one of the MEMS resonator systems 200, 300A, 300B, 300C, 300D,300E, or 500.

FIG. 8 is a conceptual diagram of an electronic device 800 configured toimplement one or more aspects of the present invention. As shown,electronic device 800 includes, without limitation, a timing signalgenerator 820 configured to provide a timing signal to applicationcircuitry 810. In one embodiment, the timing signal generator 820includes the MEMS oscillator sustaining circuit 700. As described inFIG. 7, the drive circuit 720 of the MEMS oscillator sustaining circuit700 produces the output signal 790 that is supplied to other circuitryin the timing signal generator 820 to produce the timing signal providedto the application circuitry 810. Electronic device 800 may be any typeof electronic device that includes application circuitry requiring atiming signal. Some examples of electronic device 800 include, withoutlimitation, an electronic wrist watch, a personal digital assistant, anda cellular phone.

Using FIGS. 7 and 8 as examples, in alternate embodiments, the MEMSresonator system 100B may be disposed on/in the same substrate or on/indifferent substrates than the drive circuit 720. Moreover, theapplication circuitry 810 may be disposed on/in the same substrates asthe MEMS resonator system 100B and/or the drive circuit 720. FIGS. 9Athrough 9E illustrate some of the ways to position a MEMS resonator, adrive circuit, and an application circuitry on one or more substrates.In particular, the MEMS resonator system 100B and/or the drive circuit720 and/or the application circuitry 810 may be integrated on/in thesame substrate 900, as shown on FIG. 9A, on/in different substrates 900a, 900 b and 900 c, as shown on FIG. 9B, or on/in different substrates900 d, 900 e, 900 f, 900 g, 900 h and 900 i, as shown on FIGS. 9C, 9D,and 9E. All permutations and combinations thereof are intended to fallwithin the scope of the present invention. In addition, the resonatorelectrode shields can be built in/on the same substrate as the MEMSresonator system 100B or on different substrates.

One advantage of the disclosed systems is that, among other things,undesired capacitive coupling between the resonator electrodes and thenon-active elements of the MEMS resonator may be reduced by placingresonator electrode shields between the resonator electrodes and thenon-active elements. In such a configuration, interfering signals areprevented because the resonator electrodes interact only with eitherground or the active elements of the MEMS resonator. As a result, thedeleterious effects of interfering signals are mitigated, enabling thedisclosed systems to produce higher quality timing signals in a mannerthat is simpler and more efficient relative to prior art approaches.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof. For example, in alternativeembodiments, a single sense electrode may be coupled to multiple driveelectrodes, and, similarly, a single driver electrode may be coupled tomultiple sense electrodes. Also, persons skilled in the art willrecognize that the inventive principles described herein apply equallyto differentially-coupled systems. Therefore, the scope of the presentinvention is determined by the claims that follow.

1. A system for generating a timing signal, the system comprising: amicro-electromechanical system (MEMS) resonator that includes at leastone non-active element and at least one active element, wherein, inoperation, the at least one active element is configured to oscillate ina first resonant mode; at least one drive electrode configured toreceive a first time-varying signal that causes the at least one activeelement to oscillate; at least one sense electrode configured to conducta second time-varying current in response to the oscillations of the atleast one active element; and at least one resonator electrode shieldconfigured to reduce an interfering signal associated with a timevarying current in the at least one non-active element.
 2. The system ofclaim 1, wherein the at least one non-active element comprises a supportelement or a flexure element.
 3. The system of claim 2, wherein the atleast one non-active element applies a time-varying voltage on at leastone active element.
 4. The system of claim 2, wherein the interferingsignal is an induced sense current conducted by the at least one senseelectrode and is related to the induced current in the non-activeelement.
 5. The system of claim 1, wherein the at least one non-activeelement applies a time-varying voltage on at least one active element.6. The system of claim 1, wherein the interfering signal is an inducedsense current conducted by the at least one sense electrode and isrelated to the induced current in the non-active element.
 7. A systemfor generating a timing signal, the system comprising: amicro-electromechanical system (MEMS) resonator that includes at leastone active element, wherein, in operation, the at least one activeelement is configured to oscillate in a first resonant mode; at leastone drive electrode configured to receive a first time-varying signalthat causes the at least one active element to oscillate; at least onesense electrode configured to conduct a second time-varying current inresponse to the oscillations of the at least one active element; and atleast one resonator electrode shield disposed to cover a portion of theat least one active element that prevents the active element fromoscillating in an undesired resonant mode that is different from thefirst resonant mode.